Epithelial Tissue

• There are four types of tissues: epithelial, connective, muscular and nervous.

Epithelial Tissue

• Covers body surfaces
• Lines hollow organs, body cavities, and ducts
• It forms glands.
• Epithelial tissue comprised cells arranged in continuous sheets, in single or multiple layers
• Comprised of cells closely packed and held tightly together
• Covers the external and internal surfaces of the body
• Has a free surface not covered by another tissue
• Selective barrier regulating movement in and out of the body
• Is secretory, releasing product onto the free surface
• Provides protection against the environment
• Epithelial cell surfaces differ in structure and have specialized functions
• The apical surface faces the body surface, cavity, lumen, or duct
• Lateral surfaces face adjacent cells
• The basal surface is opposite the apical layer and adheres to extracellular materials
• The basement membrane is a thin double extracellular layer for attachment to the overlying epithelial tissue
• The basal lamina is closer to, and secreted by, the epithelial cells, containing laminin, collagen, glycoproteins, and proteoglycans.
• The reticular lamina is closer to the underlying connective tissue, containing collagen secreted by connective tissue cells.
• Has its own nerve supply
• It is avascular, lacking its own blood supply
• Nutrients are delivered and waste eliminated by blood vessels present in connective tissue
• Has a high rate of cell division for renewal and repair
• Has roles in protection, filtration, secretion, absorption and excretion
• There are two types: covering and lining epithelium and glandular epithelium
• Covering and lining epithelia act as outer coverings of skin and some internal organs
• Glandular epithelium forms the secreting portion of glands like the thyroid, adrenal, and sweat glands

Covering and Lining Epithelium

• Normally classified by: arrangement of cells in layers and the shapes of its cells
• Consists of one or more layers, depending on function
• The simple form is a single layer of cells for diffusion, osmosis, filtration, secretion, or absorption
• The pseudostratified form appear to have multiple layers because cell nuclei are at different levels and all cells do not reach the apical surface
• Stratified epithelium has two or more layers of cells and protects underlying tissues in areas of wear and tear
• Cells Vary in shape depending on their function
• Squamous cells are thin, arranged like floor tiles, and allow for rapid passage of substances
• Cuboidal cells are as tall as they are wide, shaped like cubes or hexagons, may have microvilli and act in secretion or absorption
• Columnar cells are much taller than they are wide, like columns, may have cilia or microvilli and have specialized function for secretion and absorption
• Transitional cells change shape, transition for flat to cuboidal and are found in organs like the urinary bladder which stretch to larger size, and collapse to a smaller size

Simple Epithelium Sub Types

• Simple squamous epithelium
• Simple cuboidal epithelium
• Simple columnar epithelium (nonciliated and ciliated)
• Pseudostratified columnar epithelium (nonciliated and ciliated)
• Simple squamous is a single layer of cells that resembles floor tiles, with oval nuclei located in the center, and is found at sites for filtration or diffusion
• Simple cuboidal is cuboidal shaped cells with round nuclei located in the center and is found in the thyroid gland and kidneys, functioning in secretion and absorption
• Simple columnar epithilium is column shaped cells with oval nuclei near the base and is either nonciliated or ciliated

Simple Columnar Epithelium Sub Types

• Non-ciliated simple columnar epithelium contains columnar cells with microvilli at their apical surface and goblet cells
• Secreted mucus serves as lubricant for the lining of digestive, respiratory, reproductive, and urinary tracts and prevents the destruction of the stomach lining by acidic gastric juices.
• Ciliated simple columnar epithelium has columnar epithelial cells with cilia at the apical surface

Pseudostratified Columnar Epithelium

• Appears to have several layers because nuclei are at different depths
• All cells are attached to the basement membrane in a single layer but some do not extend to the apical surface
• Ciliated cells secrete mucus and bear cilia
• Nonciliated cells lack cilia and goblet cells

Stratified Epithelium

• Has two or more layers of cells
• Types determined by the shape of the cells in the apical layer
• Possible types: stratified squamous, stratified cuboidal, stratified columunar and transitional epithelium

Stratified Squamous Epithelium

• Has several layers of cells that are flat in the apical layer
• New cells are pushed up toward apical layer
• As cells move further from the blood supply they dehydrate, harden, and die
• The keratinized form has the fibrous protein keratin, and is found in superficial layers of the skin
• The nonkeratinized form does not contain keratin and is found in the mouth and esophagus

Stratified Cuboidal, Columnar and Transitional Epithelium

• Stratified cuboidal is a fairly rare type of epithelium
• It's apical layers are cuboidal in shape
• Performs functions in protection
• Stratified columnar epithelium is also very uncommon
• Columnar cells are in the apical layer only
• Has shorten, irregular shaped cells in the basal layers
• It functions in protection and secretion
• Transitional epithelium is only found in the urinary system and has a variable appearance
• In the relaxed state, cells appear cuboidal and upon stretching, cells become flattened and appear squamous
• Is ideal for hollow structures subjected to expansion

Glandular Epithelium

• Secretions, called hormones, diffuse directly into the bloodstream and maintain homeostasis
• Exocrine glands secrete products into ducts that empty onto the surfaces of the epithelium, such as the skin surface or lumen of a hollow organ
• Secretions can include mucus, sweat, oil, earwax, saliva, and digestive enzymes, for example, sudoriferous (sweat) glands

Connective Tissue

• Is the most abundant and widely distributed tissue in the body
• Has numerous functions including binding tissues together
• Supports and strengthens tissue
• Protects and insulates internal organs
• Compartmentalizes and transports substances (blood)
• Is an energy reserve and supports immune responses
• The extracellular matrix is the material located between the cells, consisting of protein fibers and ground substance
• Highly vascular, supplied with nerves, except for cartilage and tendon, which have little/no blood supply or nerves

Connective Tissue Cells

• Fibroblasts secrete fibers and components of the ground substance
• Adipocytes (fat cells) store triglycerides (fat)
• Mast cells produce histamine
• White blood cells initiate immune responses
• Neutrophil and Eosinophils are types of white blood cells
• Macrophages engulf bacteria and cellular debris by phagocytosis
• Plasma cells secrete antibodies

Connective Tissue Extracellular Matrix

• The ground substance lies between cells and fibers
• It can be fluid, semifluid, gelatinous, or calcified
• It functions to support and bind cells, store water, and allow exchange between blood and cells
• It is a complex combination of proteins and polysaccharides
• Fibers include collagen, elastic, and reticular fibers

Classification of Connective Tissues

• Types include loose connective tissue: areolar, adipose, and reticular
• Dense connective tissue: dense regular, dense irregular, and elastic
• Cartilage: hyaline, fibrocartilage, and elastic cartilage
• Bone tissue
• Liquid connective tissue: blood and lymph

Types of Loose Connective Tissues

• Areolar connective tissue is the most widely distributed in the body, and contains several cell types and all three fiber types
• Adipose tissue contains adipocytes, is good for insulation and energy reserves, and includes white (common) and brown adipose tissue
• Reticular connective tissue has fine interlacing reticular fibers (thin fibers of collagen) and reticular cells
• It forms the stroma of the liver, spleen, and lymph nodes

Dense Connective Tissue

• Dense connective tissues contains numerous, thicker, and denser fibers than others
• It is more closely packed and has fewer cells than loose connective tissue
• Dense regular connective tissue is arranged in bundles of collagen fibers are regularly arranged in parallel patterns for strength and is present in tendons and most ligaments
• In dense irregular connective tissue the collagen fibers are usually irregularly arranged
• Is found where pulling forces are exerted in many directions, in the dermis of skin and heart
• Elastic connective tissue contains branching elastic fibers
• It is strong and can recoil to its original shape after stretching, such as in lung tissue and arteries

Cartilage

• A dense network of collagen and elastic fibers firmly embedded in chrondroitin sulfate
• Chrondrocytes are cartilage cells found in the spaces called lucunae
• Has a covering of dense irregular connective tissue that surrounds it, known as the pericondrium
• Has no blood vessels or nerves, except for the pericondrium

Types of Cartilage

• Hyaline cartilage is the most abundant cartilage in the body and is surrounded by perichondrium (some exceptions; articular cartilage)
• Provides flexibility and support and reduces friction
• Fibrocartilage has chondrocytes scattered among bundles of collagen fibers within the extracellular matrix, lacks a perchondrium, and is the strongest type of cartilage
• Is found in intervertebral discs (between vertebrae).
• Elastic cartilage has chondrocytes are located within a threadlike network of elastic fibers, a pericondrium is present, and it provides strength and elasticity.
• Bone tissue is comprised of bones are organs composed of several different connective tissues: bone (osseous) tissue, periosteum, and endosteum
• Can be compact or spongy
• Has an osteon or haversian system
• Spongy bone lacks osteons but has columns called trabeculae
• Liquid connective tissue includes blood tissue, i.e.,connective tissue with liquid extracellular matrix called blood plasma; and includes lymph

Muscular Tissue

• Consists of elongated cells called muscle fibers
• Cells use ATP to generate force
• Has several functions
• It is classified into 3 types: skeletal, cardiac, and smooth muscular tissue
• Skeletal muscle tissue is attached to the skeleton (bones), has striations and it's movement or contractions are voluntary or consciously controlled
• It varies in length (up to 40 cm) and is roughly cylindrical in shape
• Cardiac muscle tissue has striations, and it's involuntary movement or contraction is not consciously controlled, also has intercalated disks
• Smooth muscle tissue contains walls of hollow internal structures, such as blood vessels, airways of lungs, the stomach, and intestines is nonstriated, and usually operates under involuntary control

Nervous Tissue

• 2 principle types of cells Neurons or nerve cells and Neuroglia

Excitable Cells

• Include neurons and muscle fibers
• Exhibit electrical excitability
• Has the ability to respond to certain stimuli by producing electrical signals like action potentials
• Action potentials propagate along a nerve or muscle plasma membrane to cause a response
• Can include a Release of neurotransmitters, and Muscle contraction